Since the advent of Hyperglide, or gated shifting, with it's cut down teeth which make shifting easier, chains have had to have peened rivets to prevent the plates from spreading when shifting. That's been 20 years or so, starting with 7 speed.
So all chains intended for 7s and more gears need to be closed with a master link or the special pin, otherwise the chain will break sometime after a hard shift under load.
If the OP is lucky, he cut the chain to the long side, and can afford to remove the damaged link, and re-close his chain properly using the pin provided. If not he'll need two pins. One at each end of a spliced in section to close the chain at the correct length.
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